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1 DISCRETE AND CONTINUOUS Website: DYNAMICAL SYSTEMS SERIES B Volume 2, Number4, November2002 pp OPTIMAL CONTROL OF TREATMENTS IN A TWO-STRAIN TUBERCULOSIS MODEL E. Jung Computer Science and Mathematics Division Oak Ridge National Laboratory, Oak Ridge, TN S. Lenhart Department of Mathematics; University of Tennessee Knoxville, TN Computer Science and Mathematics Division Oak Ridge National Laboratory, Oak Ridge, TN Z. Feng Department of Mathematics; Purdue University West Lafayette, IN (Communicated by Glenn Webb) Abstract. Optimal control theory is applied to a system of ordinary differential equations modeling a two-strain tuberculosis model. Seeking to reduce the latent and infectious groups with the resistant-strain tuberculosis, we use controls representing two types of treatments. The optimal controls are characterized in terms of the optimality system, which is solved numerically for several scenarios.. Introduction. In the absence of an effective vaccine, current control programs for TB have focused on chemotherapy. The antibiotic treatment for an active TB (with drug-sensitive strain) patient requires a much longer period of time and a higher cost than that for those who are infected with sensitive TB but have not developed the disease. Lack of compliance with drug treatments not only may lead to a relapse but to the development of antibiotic resistant TB one of the most serious public health problems facing society today. A report released by the World Health Organization warns that if countries do not act quickly to strengthen their control of TB, the multidrug resistant strains that have cost New York City and Russia hundreds of lives and more than $ billion each will continue to emerge in other parts of the world [6]. The reduction in cases of drug sensitive TB can be achieved either by case holding, which refers to activities and techniques used to ensure regularity of drug intake for a duration adequate to achieve a cure [7], or by case finding, which refers to the identification (through screening, for example) of individuals latently infected with sensitive TB who are at high risk of developing the disease and who may benefit from preventive intervention []. These preventive 99 Mathematics Subject Classification. 34C60, 34K35, 49J5, 92D30. Key words and phrases. Mathematical Model, Tuberculosis, Optimal Control, Antibiotic Resistance. 473

2 474 E. JUNG, S. LENHART, AND Z. FENG treatments will reduce the incidence (new cases per unit of time) of drug sensitive TB and hence indirectly reduce the incidence of drug resistant TB. Costs for activities to facilitate case holding and case finding may vary depending on many factors. For example, case holding can be very challenging because of the fact that chemotherapy must be maintained for several months to ensure a lasting cure, but patients usually recover their sense of well-being after only a few weeks of treatment and may often stop taking medications []. It has been reported by the Centers for Disease Control [6] that, in the United States, about 22% of patients currently fail to complete their treatment within a 2-month period and in some areas the failure rate reaches 55% [6]. In the past few years, many places in the world have adopted the DOTS (directly observed therapy strategy) in which public health nurses, community outreach workers, and others carry most of the responsibility for monitoring the patients during their course of treatment through home visits and administration. Although this program requires a relatively shorter period of time for the treatment, there was only about 24% of all TB patients who were treated through DOTS in 999 [7]. For case finding, we mainly consider actions for the prevention of disease development with preventive therapy of latently infected persons with sensitive TB. There are several case finding methods. Active case finding refers to methods for the identification of TB cases where the first initiative patient/provider contact is taken by health care providers, whereas Passive case finding refers to methods for the identification of TB cases where the first initiative patient/provider contact is taken by the patient. Another choice of case finding may be targeted screening activities among population groups at high risk of TB (immigrants from high prevalence countries, for example). Different methods have been shown to yield various levels of rewards in resource-poor and resource-rich countries (see [0] and [2]), and the amount of resources required is also different. Some past models of tuberculosis, particularly the predictive models attempting to calculate a threshold for the basic reproductive number R 0, have incorporated drug treatment and/or vaccination, and have discussed control of the disease by looking at the role of disease transmission parameters in the reduction of R 0 and the prevalence of the disease (see [] [5]). However, these models did not account for time dependent control strategies since their discussions are based on prevalence of the disease at equilibria. Time dependent control strategies have been studied for HIV models (see [8] and [9]). Both approaches of studying control strategies produce valuable theoretical results which can be used to suggest or design epidemic control programs. Depending on a chosen goal (or goals) various objective criteria may be adopted. In this article we consider (time dependent) optimal control strategies associated with case holding and case finding based on a two-strain TB model developed in [4]. This model assumes that individuals in the latent stage develop active TB at a given rate. It also assumes that a proportion of treated individuals with active TB does not finish the treatment, of which a fraction will develop drug resistant TB. We introduce into this model two control mechanisms representing case finding and case holding efforts. The case finding effort is incorporated by adding a control term that identifies and cures a fraction of latent individuals so that the rate at which latent individuals develop the disease will be reduced. The case holding effort is incorporated by adding a control term that may lower the treatment failure rate of individuals with active sensitive TB so that the incidence of acquired drug-resistant TB will be reduced. Our objective functional balances the effect of minimizing

3 A TWO-STRAIN TUBERCULOSIS MODEL 475 the cases of latent and infectious drug-resistant TB and minimizing the cost of implementing the control treatments. This paper is organized as follows: Section 2 describes a two-strain TB model with two control terms. Our objective functional is also introduced in this section. The analysis of optimal controls is given in Section 3. Section 4 includes some numerical studies of optimal controls and discusses our results. 2. A Two-strain TB Model. Our state system is the following system of six ordinary differential equations from [4]: Ṡ =Λ β S I N β S I 2 N µs L = β S I N (µ + k )L u (t)r L +( u 2 (t))pr 2 I + β 2 T I N I 2 β L N I = k L (µ + d )I r 2 I L 2 =( u 2 (t))qr 2 I (µ + k 2 )L 2 + β (S + L + T ) I 2 N I 2 = k 2 L 2 (µ + d 2 )I 2 T = u (t)r L +( ( u 2 (t))(p + q))r 2 I β 2 T I N β T I 2 N µt with S(0),L (0),I (0),L 2 (0),I 2 (0),T(0) given, where the host population is divided into the following epidemiological classes (state variables): S: Susceptible L : Latent, infected with typical TB but not infectious I : Infectious with typical TB L 2 : Latent, infected with resistant strain TB but not infectious I 2 : Infectious with resistant strain TB T : Treated (effectively), N = S + L + I + L 2 + I 2 + T. We assume that an individual may be infected only through contacts with infectious individuals. Λ is the recruitment rate. β and β 2 are the rates at which susceptible and treated individuals become infected by an infectious individual with typical TB, respectively. β is the rate at which an uninfected individual becomes infected by one resistant-tb infectious individual. The per-capita natural death rate is µ while the per-capita disease induced death rates are d and d 2 for the typical TB and resistant TB, respectively. The rates at which an individual leaves the two latent classes by becoming infectious are k and k 2. r and r 2 are the treatment rates of individuals with latent and infectious typical TB, respectively, and p + q is the proportion of those treated infectious individuals who did not complete their treatment (p + q ). The control functions, u (t) andu 2 (t), are bounded, Lebesgue integrable functions. The case finding control, u (t), represents the fraction of typical TB latent individuals that is identified and will be put under treatment (to reduce the number of individuals that may be infectious). The coefficient, u 2 (t), represents the effort that prevents the failure of the treatment in the typical TB infectious individuals (to reduce the number of individuals developing resistant TB). When ()

5 A TWO-STRAIN TUBERCULOSIS MODEL 477 with transversality conditions λ i (t f )=0,i=,...,6 (6) and N = S + L + I + L 2 + I2 + T. The following characterization holds and u (t) =min(max(a, u 2(t) =min(max(a 2, B (λ 2 λ 6 )r L ),b ) B 2 (λ 2 p + λ 4 q λ 6 (p + q)r 2 I )),b 2 ). Proof. Corollary 4. of [3] gives the existence of an optimal control pair due to the convexity of integrand of J with respect to (u,u 2 ), a priori boundedness of the state solutions, and the Lipschitz property of the state system with respect to the state variables. Applying Pontryagin s Maximum Principle, we obtain dλ dt = H S,λ (t f )=0,... dλ 6 = H dt T,λ 6(t f )=0, evaluated at the optimal control pair and corresponding states, which results in the stated adjoint system (5) and (6), [4]. By considering the optimality conditions, H =0, H =0 u u 2 and solving for u, u 2, subject to the constraints, the characterizations (7) can be derived. To illustrate the characterization of u,wehave H u = B u λ 2 r L + λ 6 r L =0 at u on the set {t a <u (t) <b }. On this set, (7) u (t) = B (λ 2 λ 6 )r L. Taking into account the bounds on u, we obtain the characterization of u (7). in Due to the a priori boundedness of the state and adjoint functions and the resulting Lipschitz structure of the ODEs, we obtain the uniqueness of the optimal control for small t f. The uniqueness of the optimal control pair follows from the uniqueness of the optimality system, which consists of () and (5), (6) with characterizations (7). There is a restriction on the length of the time interval in order to guarantee the uniqueness of the optimality system. This smallness restriction on the length on the time interval is due to the opposite time orientations of (),

6 478 E. JUNG, S. LENHART, AND Z. FENG (5), and (6); the state problem has initial values and the adjoint problem has final values. This restriction is very common in control problems (see [8] and [9]). Next, we discuss the numerical solutions of the optimality system and the corresponding optimal control pairs, the parameter choices, and the interpretations from various cases. 4. Numerical Results. In this section, we study numerically an optimal treatment strategy of our two-strain TB model. The optimal treatment strategy is obtained by solving the optimality system, consisting of 2 ODEs from the state and adjoint equations. An iterative method is used for solving the optimality system. We start to solve the state equations with a guess for the controls over the simulated time using a forward fourth order Runge-Kutta scheme. Because of the transversality conditions (6), the adjoint equations are solved by a backward fourth order Runge-Kutta scheme using the current iteration solution of the state equations. Then, the controls are updated by using a convex combination of the previous controls and the value from the characterizations (7). This process is repeated and iteration is stopped if the values of unknowns at the previous iteration are very close to the ones at the present iteration. For the figures presented here, we assume that the weight factor B 2 associated with control u 2 is greater or equal to B which is associated with control u. This assumption is based on following facts: The cost associated with u will include the cost of screening and treatment programs, and the cost associated with u 2 will include the cost of holding the patients in the hospital or sending people to watch the patients to finish their treatment. Treating an infectious TB individual takes longer (by several months) than treating a latent TB individual. In these three figures, the set of the weight factors, B =50andB 2 = 500, is chosen to illustrate the optimal treatment strategy. Other epidemiological and numerical parameters are presented in Tables and 2, respectively. We will discuss briefly the cases with different values of B and B 2 later in this section. Figure shows the optimal treatment strategy for the case of B =50and B 2 = 500. In the top frame, the controls, u (solid curve) and u 2 (dashdot curve), are plotted as a function of time. In the bottom frame, the fractions of individuals infected with resistant TB, (L 2 + I 2 )/N, with control (solid curve) and without control (dashed curve) are plotted. Parameters N = and β = are chosen. Other parameters are presented in Tables and 2. To minimize the total number of the latent and infectious individuals with resistant TB, L 2 + I 2,the optimal control u 2 is at the upper bound during almost 4.3 years and then u 2 is decreasing to the lower bound, while the steadily decreasing value for u is applied over the most of the simulated time, 5 years. The total number of individuals L 2 + I 2 infected with resistant TB at the final time t f = 5 (years) is 23 in the case with control and 476 without control, and the total cases of resistant TB prevented at the end of the control program is 3053 (= ). Figure 2 illustrates how the optimal control strategies depend on the parameter β, which denotes the transmission rate of primary infections of resistant TB. The value of β is usually given by the product of the number of contacts (with an infectious individuals with resistant TB) per person per unit of time and the probability of being infected with resistant TB per contact. This value varies from

7 A TWO-STRAIN TUBERCULOSIS MODEL Controls u u (L2+I2)/N with control (L2+I2)/N without control 0.2 (L2+I2)/N Time (year) Figure. B 2 = 500. The optimal control strategy for the case of B =50and Controls u:beta*=0.03 u2:beta*= u:beta*=0.027 u2:beta*=0.027 u:beta*= u2:beta*=0.029 u:beta*= u2:beta*= Time (years) Figure 2. The controls u are plotted as a function of time for the 4 different values of β, 0.03, 0.027, , and and the only one control u 2 (top curve) is plotted because u 2 remains almost the same as β increases. place to place depending on many factors including living conditions. In Figure 2, the controls, u (dark color curves) and u 2 (light color curves), are plotted as a function of time for the 4 different values of β, 0.03, 0.027, , and

8 480 E. JUNG, S. LENHART, AND Z. FENG Control u N = N = 6000 N = Control u N = 2000 N = N = Time (years) Figure 3. The controls, u and u 2, are plotted as a function of time for N = 6000, 2000, and in the top and bottom frame, respectively. These values for β are chosen from [4]. Other parameters are presented in Tables and 2. Figure 2 shows that u plays an increasing role while u 2 remains almost the same as β decreases (that is why only one u 2 graph is shown). This is an expected result because when β is smaller, the new cases of resistant TB arise more from infections acquired from L and I due to treatment failure than from primary infections. In this case, identifying and curing latently infected individuals with sensitive TB becomes more important in the reduction of new cases of resistant TB. In Figure 3, the controls, u and u 2, are plotted as a function of time for N = 6000, 2000, and in the top and bottom frame, respectively. Other parameters except the total number of individuals and β = are fixed for these three cases and presented in Tables and 2. These results show that more effort should be devoted to case finding control u if the population size is small, but case holding control u 2 will play a more significant role if the population size is big. Note that, in general, with B fixed, as B 2 increases, the amount of u 2 decreases. A similar result holds if B 2 is fixed and B increases. In conclusion, our optimal control results show how a cost-sffective combination of treatment efforts (case holding and case finding) may depend on the population size, cost of implementing treatments controls and the parameters of the model. We have identified optimal control strategies for several scenarios. Control programs that follow these strategies can effectively reduce the number of latent and infectious resistant-strain TB cases. Acknowledgments. E. Jung s research was supported by the Applied Mathematical Sciences subprogram of the Office of Science Research, U.S. Department

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